This invention relates to a novel method of metallizing a phosphur screen and particularly to such a method which uses an aqueous emulsion of acrylic copolymers in a particular compositional range.
A proces of metallizing a phosphor screen for a cathode-ray tube is described in U.S. Pat. No. 3,067,055 issued on Aug. 5, 1959, to T. A. Saulnier, Jr. That process includes the steps of coating the screen with an aqueous emulsion containing an alkyl methacrylate-methacrylic acid copolymer, heating and drying the coating to produce a dry volatilizable substrate, depositing a layer of metal on the surface of the substrate and then volatilizing the substrate, leaving the metal layer in contact with the screen. In addition to the copolymer, the emulsion may contain minor amounts of one or more additives such as colloidal silica, a boric acid complex of polyvinyl alcohol and hydrogen peroxide, as described and for the reasons disclosed, for example, in U.S. Pat. No. 3,582,390 issued on June 1, 1971 to T. A. Saulnier.
As used herein, the combination of steps for producing the substrate is referred to as "filming," and the particular filming process described above is referred to as "emulsion filming." The emulsion used for coating the screen is called the "filming emulsion." The filming emulsion has as its major constituent a "latex" whose discontinuous phase consists essentially of particles of an organic copolymer. The step of volatilizing the substrate is called "baking-out."
While emulsion filming has been used successfully for the manufacture of millions of color television picture tubes, it has the disadvantage that very few latexes are known which can be successfully employed to prepare the filming emulsion. In fact, virtually all kinescope manufacturers that employ emulsion filming base their filming emulsions on a commercial product known variously as Rhoplex B-74 and Primal B-74. It is believed that both of these products are substantially identical latexes. However, the components of both of these products are maintained as trade secrets, and routine analyses and the application of ordinary skill have failed to provide duplicate or alternative products.
The trade-secret nature of these commercial latexes is a detriment, particularly on those occasions when, due to quality variation or other reasons, the supply of usable material diminishes. Even when a steady supply of satisfactory commercial latex is available, the emulsion filming process itself must be performed within rather rigid limits. For example, yield from emulsion filming is quite sensitive to such parameters as: total non-volatile concentration in the filming emulsion, pH of the incoming latex, temperature of the screen when the emulsion is dispensed, heating and drying conditions, the thickness of the dry coating and the baking-out conditions.
Should commercial latexes become unavailable, it is desirable to have an alternative source of latex that could be used. Other known filming methods cannot be substituted easily, since they employ organic-solvent-based polymers and therefore require special safety equipment that is not required or used with emulsion filming.
An important factor in successful emulsion filming is the composition of the filming emulsion, and its single most important component is the latex that is used. Thus, it is desirable to provide an emulsion-filming latex having a specifically-identified composition which can be quality controlled by chemical or instrumental analyses rather than by trial-and-error experiments.